DIYARBAKIR, Turkey, Jan 5 (Reuters) - Turkish security forces have killed at least 14 militants in the mainly Kurdish southeast, the military said on Tuesday, as an army campaign to quell fighting that has spread to the streets of cities entered a third week.

A member of the "village guard", a Kurdish militia fighting alongside government forces, was killed on Tuesday during clashes with Kurdistan Workers Party (PKK) fighters in the district of Sur in Diyarbakir, the regional capital, the General Staff said on its website.

The PKK members were killed in Sur and the towns of Cizre and Silopi on Monday, the military said. Those areas have been under round-the-clock curfews since last month.

It said a total of 296 rebels had been killed since Dec. 14, when security forces intensified action against the PKK. The PKK has moved to urban areas, building trenches and barricades to keep police and soldiers at bay.

Dozens of civilians and soldiers have also died.

The PKK's 2-12/-year ceasefire collapsed in July with the worst violence in two decades.

The autonomy-seeking PKK, deemed a terrorist group by the United States and the European Union as well as Turkey, took up arms in 1984. More than 40,000 people, mainly Kurds, have died.

Separately, Turkish authorities re-opened a key border gate with Iraq near Cizre and Silopi during daylight hours, customs officials said. Thousands of Turkish drivers had been stuck on the Iraqi side of the crossing after Turkey shut the Habur crossing on Dec. 14 for security reasons.

In Diyarbakir, two senior officials in the pro-Kurdish Democratic Regions Party (DBP) were detained and party offices raided, witnesses said. The reason for the detentions was not immediately known. (Reporting by Seyhmus Cakan and Melih Aslan; Writing by Ayla Jean Yackley; editing by Ralph Boulton)